This invention relates to an apparatus for automatically processing flat articles and more particularly, to an apparatus for feeding flat articles such as letters, postcards, and paper sheets.
In a feeding apparatus, a large number of flat articles are stored inside a hopper section while being kept upright and are transferred to a feed mechanism section, where the flat articles are sequentially fed out one by one to a next processing apparatus. Such an apparatus is disclosed in German Pat. No. 1,217,405.
As the flat articles are sequentially fed out one by one, the packing density of the flat articles inside the hopper section changes to be more sparse. This change is detected and the remaining flat articles in the hopper section are transferred to the feed mechanism section in accordance with this change in order to keep the packing density either constant or within a predetermined range. The packing density of the flat articles has been conventionally detected by means such as a micro-switch with a push force detection lever that detects the pressure of the flat articles being transferred to the feed mechanism section. This pressure is regarded as the packing density of the flat articles and is converted into an electric signal to perform an ON/OFF control of the transfer of the flat articles to the feed mechanism section in accordance with the electric signal.
Especially when the flat articles to be processed are mail items such as post cards and letter envelopes, they are remarkably different from each other in the paper quality, hardness and softness, thickness, weight, and so forth. These differences are all the more enhanced by the contents of envelopes. Accordingly, the conventional practical apparatus must adopt the pushing force delivered from flat articles having an average hardness as the reference in order to judge the packing density of the flat articles.
In case there is a high ratio of hard flat articles to normal or soft articles in the hopper section, the push force detection lever connected to the micro-switch is inertialy urged. This means that gaps are formed between the flat articles at the front part of the feed mechanism section and the number of the flat articles in the hopper section is reduced. Then, push force detection lever can not easily return to its original position so that the flat articles can not be transferred to the feed mechanism section. This results in a reduction of feed accuracy and in feed interruption, causing eventually a reduction of the processing speed. The interrupted feed also increases noise. To the contrary, if the ratio of softer or lighter flat articles such as air-entrapping envelops is great, the push force detection lever can not be pushed even if the packing density of the flat articles is high. As a consequence, the flat articles are excessively transferred to the feed mechanism section and the force of friction between the flat articles increases so that they can not be correctly fed one by one, but they are incorrectly fed as two or more flat articles simultaneously.
To prevent a simultaneous feeding of two or more flat articles, the conventional apparatus of the kind described above is equipped with a vacuum suction sub-chamber positioned beside the feed mechanism section to cause the next flat article to cling owing to the suction. However, the degree of air-permeation varies depending upon the kind and quality of the flat articles. Hence, the push force detected on the side of the feed mechanism section does not always represent the correct packing density between the flat articles. In other words, the flat articles having less air-permeation and, therefore, the suction is stronger owing to the negative pressure of the vacuum suction sub-chamber than the flat articles having high air-permeation. The push force of the flat articles becomes great even if large gaps exist between the remaining flat articles.
Hence, in the prior art, the micro-switch with the push force detection lever can not detect the pressure corresponding to the actual packing density of the flat articles and the feed of the flat articles can not be effected correctly.